Lock API doc: Fix error message for delete lock forbidden

Looks like it was a copy and paste from the "verify lock" example.

Change-Id: I17d2799e4bf3ad27b45816bd78033ea1ed636faf
Signed-off-by: David Pursehouse <dpursehouse@collab.net>

docs/api/locking.md

@@ -422,7 +422,7 @@ errors.
 // HTTP/1.1 403 Forbidden
 // Content-Type: application/vnd.git-lfs+json
 {
-  "message": "You must have push access to verify locks",
+  "message": "You must have push access to delete locks",
   "documentation_url": "https://lfs-server.com/docs/errors",
   "request_id": "123"
 }